According to a conventional air refrigeration system, an air refrigerant cooling apparatus which is a component of the air refrigeration system uses a motor in which a rotor shaft is rotated on bearings such as ball bearings, roller bearings and so on. In order to generate cooling air with a desired temperature, it is necessary to provide not only the air refrigerant cooling apparatus but also an air cooler and a booster for compressing air refrigerant supplied to the air refrigerant cooling apparatus beforehand. Also, with regard to the motor using the ball bearings the roller bearings and the like, maintenance for replacing such the bearings is necessary with a periodic interval. It is not possible during the maintenance to cool freights in a warehouse.
As a means for improving refrigeration performance of the air refrigerant cooling apparatus and simplifying a configuration of the air refrigeration system, high-speed rotation can be considered in which the rotational frequency of the motor in the air refrigerant cooling apparatus is increased.
A “magnetic bearing apparatus” is popularly known as a driving mechanism for attaining the high-speed rotation. FIG. 1 shows a schematic configuration of a “magnetic bearing apparatus” disclosed in Japanese Unexamined Patent Publication JP-P-Heisei 8-61366. The magnetic bearing apparatus shown in FIG. 1 has an axial magnetic bearing 5 for controlling axial displacement of a shaft 2 through a rotor disk 4 with a disk shape fixed to the shaft 2, and a pair of radial magnetic bearings 6 and 7 which located on both sides of the axial magnetic bearing 5 in the axial direction and controls radial displacement of the shaft 2. In addition, the magnetic bearing device has an annular member surrounding the circumference of the rotor disk 4, a vent 13a provided in the annular member for blowing an air to cool down the rotor disk, and a couple of gap providing members 20 and 21. The gap providing members 20 and 21 surround the circumference of the shaft 2 at the both sides of the rotor disk 4 in the axial direction, which forms substantially constant gaps between the shaft 2 and the gap providing members for passing the air from the vent 13a. 
Also, in association with the technology mentioned above, another technology is proposed as shown below.
An “air refrigerant cooling apparatus” disclosed in Japanese Unexamined Patent Publication JP-P-Heisei 11-132582 is characterized by a compressor, an air cooler, an air-to-air heat exchanger, and an expander arranged along the passage of air for conveying the air from a cooling room to be chilled through the air-to-air heat exchanger to the compressor and feeding the cooling room to be chilled with the air released from the expander. The air refrigerant cooling apparatus includes a first valve-equipped bypass for allowing a portion or all of the air released from the expander to bypass the cooling room to be chilled and then return back to the air-to-air heat exchanger, and a valve-equipped hot-air bypass for introducing air at not lower than 0° C. from the path between the compressor and the expander and supplying the air to an inlet air path of the air-to-air heat exchanger.